A polarizing plate having a function to transmit or shield light is, along with liquid crystals which have a function of switching light, a fundamental constituent element of a display apparatus such as a liquid crystal display (LCD). The area of application of this LCD has expanded broadly from the early day small items such as an electronic calculator and a watch to a notebook computer, a word processor, a liquid crystal projector, a liquid crystal television, a car navigation system, and indoor and outdoor measurement instruments. Also, the LCD is used in broad conditions from low to high temperature, from low to high humidity, and from low to high light intensity. Thus, a polarizing plate having high polarization performance and excellent durability is desired.
At present, the polarizing film is produced by dyeing a polarizing film substrate with or incorporating therein iodine or a dichroic dye as a polarizing element, where the substrate is a stretched and oriented film of polyvinyl alcohol or its derivative, or an oriented film of polyene prepared by dehydrochlorination of a polyvinyl chloride film or dehydration of a polyvinyl alcohol film. Among these, an iodine polarizing film which uses iodine as the polarizing element exhibits superior polarization performance. However, this polarizing film is weak to moisture and heat, and when it is used for a long time under conditions of high temperature and high humidity, a problem of durability arises. In order to improve durability, methods such as treating the polarizing film with formalin or an aqueous solution containing boric acid and using a polymer film of low moisture permeability as a protective film are considered. However, the effects of these methods are not satisfactory. On the other hand, a dye-containing polarizing film comprising a dichroic dye as the polarizing element has better moisture resistance and heat resistance than the iodine polarizing film, but, generally, is insufficient in polarization performance.
In a neutral color polarizing film produced by adsorbing several kinds of dichroic dyes to a polymer film followed by orientation, if there is light leakage (color leakage) of a specific wavelength in the wavelength range of visible light, in a state (the perpendicular position) where two polarizing films are superimposed on each other in such a way that their orientation directions are perpendicular to each other, the hues of the liquid crystal display may change in the dark state when the polarizing films are fixed to the liquid crystal display panel. Thus, in order to prevent the discoloration of the liquid crystal display due to color leakage of a specific wavelength in the dark state when the polarizing film is fixed to a liquid crystal display apparatus, it is necessary to uniformly lower the transmittance at the perpendicular position (perpendicular transmittance) in the wavelength range of visible light.
Further, in a case of a color liquid crystal projection type display, namely, a color liquid crystal projector, a polarizing plate is used for the liquid crystal image-forming part. In this application, the iodine polarizing plate was used previously, which has good polarization performance and exhibits neutral gray color. However, as mentioned above, the iodine polarizing plate has a problem that its light resistance, heat resistance, and wet heat resistance are not sufficient, because iodine is the polarizer. In order to solve this problem, a neutral gray polarizing plate using a dye type dichroic colorant as the polarizer has come to be used. In the neutral gray polarizing plate, usually colorants of three primary colors are used in combination in order to improve transmittance in the entire wavelength range of visible light and polarization performance evenly. Thus, there is a problem that, to meet the demand of the marketplace for more brightness as in the color liquid crystal projector, the light transmittance is still poor and, in order to realize brightness, it is necessary to increase intensity of the light source. In order to solve this problem, three polarizing plates corresponding to the three primary colors, namely, plates for each of the blue channel, the green channel, and the red channel have come to be used.
However, decrease in brightness cannot be avoided because light is absorbed considerably by the polarizing plate and an image of such a small area as 0.5 to 3 inches is magnified to about several tens to one hundred and tens of inches. Therefore, as the light source, one of high luminance is used. Furthermore, desire for further increase in brightness of the liquid crystal projector is strong and, as a result, the intensity of the light source used is inevitably growing stronger. Along with this, the amounts of light and heat the polarizing plate receives are increasing.
A dye which may be used for producing the dye-containing polarizing film as mentioned above includes the water-soluble azo compounds described, for example, in Patent Documents 1 to 6.
However, conventional polarizing plates comprising the water-soluble dye have not yet satisfied the market needs sufficiently in terms of polarization characteristics, the range of absorption wavelength, hues, and the like. Furthermore, among three polarizing plates corresponding to the three primary colors for a color liquid crystal projector, namely, the plates for each of the blue channel, the green channel, and the red channel, none is good in all of brightness, polarization performance, durability under a condition of high temperature and high humidity, and resistance to prolonged irradiation of light. Thus, improvement is desired.    PATENT DOCUMENT 1: JP 2001-33627 A    PATENT DOCUMENT 2: JP 2001-56412 A    PATENT DOCUMENT 3: Japanese Patent No. 2622748    PATENT DOCUMENT 4: JP 2004-51645 A    PATENT DOCUMENT 5: WO 2004/092282    PATENT DOCUMENT 6: WO 2006/057214    NON-PATENT DOCUMENT 1: “Dyestuff Chemistry” by Yutaka Hosoda